1. Field of the Invention
The present invention relates to a developing apparatus for an electrostatic latent image, used with a copying machine, printer, facsimile, displaying system and the like.
2. Related Background Art
In a developing apparatus using two-component developer including carrier particles and toner particles, since the toner particles are used during each developing cycle, the toner density in the developer is gradually decreased as the developing cycles increase. In order to keep the toner density in the developer at a constant value, the toner density in the developer being used in the developing apparatus must always be detected exactly and new toner particles must be replenished on the basis of the detection result.
Therefore, in order to detect the toner density in the developer being used in the developing apparatus, Japanese Patent Laid-Open No. 53-107853 and U.S. Pat. No. 4,985,823 disclose a developing apparatus as shown in FIG. 1.
In FIG. 1, the reference numeral 1 denotes an electrophotographic photosensitive drum rotating in a direction shown by the arrow A; reference numeral 2 denotes a magnet roller arranged in parallel with the photosensitive drum 1; reference numeral 3 denotes a developing sleeve rotatably mounted around the magnet roller 2 and rotated in a direction B in synchronism with the photosensitive drum 1 by means of a driving mechanism (not shown); reference numeral 4 denotes a developer container for containing developer D consisting of toner particles and magnetic carrier particles; reference numeral 5 denotes a screw-type agitator for agitating the developer D in the developer container 4; reference numeral 6 denotes a toner replenishing bath for containing toner particles T to be replenished into the developer container 4; reference numeral 7 denotes a screw feeder for replenishing the toner particles T in the toner replenishing bath 6 into the developer container 4; and reference numeral 8 denotes a motor for rotating the screw feeder 7.
A toner density sensor 9 has the following elements 9a-9e. The reference numeral 9a denotes a body of the toner density sensor 9 having a transparent glass window 9b disposed above the developing sleeve 3 and extending in a tangential direction (horizontal direction) to the developing sleeve 3; 9c denotes a light emitting elements such as an infrared ray LED; and 9e denotes a light receiving element. The reference numeral 10 denotes a control circuit for the light emitting element; reference numeral 11 denotes a differential amplifier; and reference numeral 12 denotes a reference value generator (reference voltage source).
In the arrangement shown in FIG. 1, when the developer adhered on the developing sleeve 3 reaches below the toner density sensor 9, the infrared ray emitted from the light emitting element 9c is illuminated on the developer through the transparent glass window 9b. The light reflected by the developer on the developing sleeve 3 is received through an infrared ray permeable filter 9d by the light receiving element 9e, where the light is converted into an electrical signal. The electrical signal is compared with the reference value in the differential amplifier 11, and the signal is amplified by the difference between the signal value and the reference value. The amplified signal is used as a control signal for driving the motor 8.
By the way, a developing apparatus which can reduce or suppress the deterioration of the image due to the sweeping traces by means of the magnetic brush and/or deterioration of toner particles has been proposed, for example, as disclosed in U.S. Pat. No. 4,933,254. In this developing apparatus, the two-component developer consisting of the toner particles and the magnetic carrier particles is born on the developing sleeve as a thin layer, and the electrostatic latent image formed on the photosensitive member is brought into the developing station. And, the developing efficiency is improved by applying the AC bias voltage superimposing the DC voltage to the developing sleeve, and the alternate electric field is generated in the developing station in order to obtain an image having high quality. Incidentally, a toner layer regulating blade is arranged in the vicinity of the developing sleeve for forming the thin developer layer on the developing sleeve. When the AC bias voltage is applied to the developing sleeve, the flying of the toner is caused at the position of the regulating blade and the discharge phenomenon at high peak value is generated. In order to eliminate such inconvenience, the AC bias voltage same as that applied to the developing sleeve must also be applied to the regulating blade, as disclosed in U.S. Pat. No. 4,395,476.
On the other hand, since the change in the toner density leads in the deterioration of the image quality regarding the image density and/or the fog in the image, in order to control the image quality precisely, it is desirable that the toner density sensor is disposed so as to detect the toner density in the developer at a position near the developing station where the toner is supplied to the latent image. To achieve this, it is preferable that the toner density sensor is arranged in the vicinity of the toner layer regulating blade (the toner passing through this blade can reach the developing station). However, if the toner density sensor is arranged in the vicinity of the regulating blade, under the influence of the AC bias voltage, a dielectric current is generated in the electric signal from the toner density sensor, thus causing the noise and not providing the correct detection signal. As a result, it was impossible to control the toner density correctly, thereby causing the deterioration of the image quality.